The invention relates to a plate-type heat exchanger and a method for joining the plates to the heat exchanger housing. More specifically, an alloy is spray welded onto the plates and the housing frame. The plates are then welded to the housing frame by standard welding methods.
The concept of plate-type heat exchangers is well-known and is commonly used for achieving an indirect heat transfer between two circulating fluids which are at different temperatures. These heat exchangers generally consist of a core containing a plurality of spaced parallel plates welded or formed to define parallel passages. Manifolds are attached to the ends of the passages to direct each fluid to alternate passages so that each plate forms a heat conducting interface between the two fluids. Heat exchangers of this type are expensive to manufacture and present a risk of the two fluids mixing through leakage.
When metals are welded together much heat is generated. Both the arc created by the weld process and the filler metal used in the weld process radiate heat out into the surrounding plates and frame metals. The radiating heat causes considerable uneven expansion of the metals. As the filler metal is deposited on the plates and frame, the filler metal begins to cool and contract. Simultaneously, the heat of the arc weld process causes expansion immediately in front of this contraction. The temperatures of the plates and frame adjacent the weld rise, causing the metal of the plates and frame to expand. As the portion of the plates and frame adjacent the weld cool, the metal contracts. This uneven rate of expansion and contraction causes stress and distortion to the metals.
There is a greater problem when metals of different thicknesses are welded together. In a heat exchanger the plates must be sufficiently thin to allow effective transfer of heat across the surface of the plates; however, the metal frame members must be thick enough to support the weight of the core. The heat radiating from the deposit of the filler metal causes a greater stress on the thinner metal of the plates than on the thicker metal of the frame members. This stress is greatest in the area immediately adjacent the deposit of the filler metal. Often the stress is great enough to cause the thinner plate metal to crack allowing leakage of the two circulating fluids between the parallel passages.
The plates are not repaired easily if cracks develope in the thin plate metal.
Further, when the arc weld process is performed the arc drawn between the plates and frame, and the electrode causes the metals in the arc stream to melt. There is a depression created in the metals immediately in front of the deposited filler metal. The depth of the depression is dependent upon the rate of deposit of the filler metal. The rate of deposit of the filler metal affects the penetration of the arc into the metals. If the rate of deposit in the arc weld process is too slow or too fast there is a risk that the arc will completely penetrate the thinner metal of the plate and form a hole in the plate. In practice the weld heat neccessary to make a penetrating weld to the frame material will melt away the thin sheets and make welding them leak tight virtually impossible.
Thus, there is a need for a plate-type heat exchanger which is more economical to manufacture and which allows a more efficient transfer of heat. There is a further need for a method for joining the plates to the frame of the heat exchanger which eliminates the problem of distortion and cracks in the thin metal plates.